There are already known various constructions of actuators, among them such that are suited for use in an array of such actuators for acting on a flexible mirror to change its shape or contour. Experience with the conventional constructions of such actuators has shown, however, that they leave much to be desired for certain applications.
So, for instance, in the field of high-power lasers having large beam diameters, it is common for the quality of the optical beam emerging from the laser gain medium to be inadequate for the purposes on hand, especially since the wave front is not ideal, that is, phase shifts exist between different regions of the cross section of the laser beam. To deal with this situation, it has already been proposed to aim the emerging laser beam against a reflecting surface of a thin flexible mirror faceplate for the laser beam to be reflected by the reflective surface, and to adjust the contour or shape of the thin flexible faceplate usually by means of a plurality of actuators which extend forward and backward and thus change the shape and contour of the flexible plate and thus the contour of the reflective surface of the mirror. The deformable mirror can be used to alter the wave front of the laser beam to allow the laser beam to be propagated more easily through the medium into which it is ultimately injected for propagation, such as through the atmosphere.
In order to be able to correct for optical phase distortions to a high energy laser (HEL) beam as it is propagated through the atmosphere, it has been established that there is a pronounced need for the use of deformable mirrors acted upon and deformed by closely packed actuators, which may have to be spaced at less than 1 centimeter apart. With the advent of more powerful HELs, the beam size, of necessity, increases, resulting in the need for using very large numbers of closely packed actuators.
The closely packed actuator array causes difficulties in the employment of replaceable actuator design features. This is so because the required small spacing of the actuators does not offer reasonable sizing of the necessary mechanical features as executed in state-of-the-art designs. Moreover, heretofore known replaceable actuator designs are known to possess deficiencies with respect to current and future needs, particularly for short wavelength devices.
With the shift to shorter wavelengths (visible and even the ultraviolet) in current and future beam control system designs, actuator performance has become of paramount importance in the development of large aperture, high actuator density (often more than 1000 actuators) deformable mirror assemblies. Replaceable actuators which provide high precision linear response with high bandwidth and close packing density capability are primary components in adaptive optical systems.
Magnetostrictive actuator designs developed over the course of the past few years already addressed and dealt with many of the issues associated with this new generation of deformable mirror architecture. However, several engineering issues have not been satisfactorily resolved by such actuator constructions. So, for instance, thermal expansion associated with iron-based materials necessitates thermal growth control. Thus, additional stroke to compensate for residual thermal growth, increased control system complexity to ensure precision response, and liquid cooling of the actuators and other parts of the mirror assembly to remove the solenoidal and other heating, were some of the approaches taken in the past to address the thermal expansion problem. Yet, such design and fabrication complexities greatly limit the scaling potential and long-term reliability of existing magnetostrictive actuator constructions and systems.
With the increased power densities associated with the new generation laser devices, such as the excimer and free electron lasers, highly reflective coatings which minimize the absorbed power are essential to mirror performance and survivability. Multi-layer dielectric coatings that offer such performance are usually deposited at about 170.degree. C. for extended soak periods. This calls for actuator constructions that are able to withstand such demanding conditions with minimal effect on the heat exchanger and on the substrate/actuator attachment interfaces.
The actuator spacing has been limited in the currently known and used magnetostrictive actuator systems by the necessity for as large a solenoidal copper volume as possible to provide high applied magnetic field and to minimize I.sup.2 R losses. This has caused problems in increasing the actuator packing density, that is in reducing the spacings between the adjacent actuators of the actuator array.
As mentioned previously, active cooling to control thermal growth greatly increases the complexity of the actuator array design. Requirements for precision machining, the use of corrosion inhibitors, and the use of specialized adhesives have also resulted in an increase in the per channel actuator costs. The additional requirement for electronic feedback compensation to control or deal with residual thermal growth also increases the electronic driver costs. Thus, it may be seen that the heretofore proposed actuator designs and systems have many drawbacks that limit their use in deformable mirror assemblies and/or increase the cost of their use in such assemblies.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide an actuator arrangement for use in deformable mirror assemblies, which does not possess the disadvantages of the known actuator arrangements of this kind.
Still another object of the present invention is so to develop the actuator assembly of the type here under consideration as to be compatible with very close packing of the actuators and relatively large surface deformations between adjacent actuators.
It is yet another object of the present invention to devise a mirror arrangement of the above type which renders it possible to deal in a particularly simple way with thermal expansion effects on the performance of the actuator arrangement and of the mirror assembly which employs such actuators.
An additional object of the present invention is to design the above mirror arrangement in such a manner as to be scalable to very large mirror sizes without performance loss or geometry changes.
A concomitant object of the present invention is construct the mirror arrangement of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.